Inkjet printing mechanisms use cartridges, often called "pens," which eject drops of liquid colorant, referred to generally herein as "ink," onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, ejecting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a "service station" mechanism is supported by the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which substantially seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit that draws a vacuum on the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as "spitting," with the waste ink being collected in a "spittoon" reservoir portion of the service station. After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead. The wiping action is usually achieved through relative motion of the printhead and wiper, for instance by moving the printhead across the wiper, by moving the wiper across the printhead, or by moving both the printhead and the wiper.
To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide quicker, more waterfast printing with darker blacks and more vivid colors, pigment-based inks have been developed. These pigment-based inks have a higher solid content than the earlier dye-based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to form high quality images on readily available and economical plain paper, as well as on recently developed specialty coated papers, transparencies, fabric and other media. Unfortunately, the combination of small nozzles and quick drying ink leaves the printheads susceptible to clogging, not only from dried ink and minute dust particles or paper fibers, but also from the solids within the new inks themselves. Partially or completely blocked nozzles can lead to either missing or misdirected drops on the print media, either of which degrades the print quality. Thus, keeping the nozzle face plate clean becomes even more important when using pigment based inks, because they tend to accumulate more debris than the earlier dye based inks.
Indeed, keeping the nozzle face plate clean for cartridges using pigment based inks has proven quite challenging. With the earlier dye-based inks, wiping the printhead with an elastomeric wiper was sufficient. However, with the advent of the pigment-based inks, a secondary operation of cleaning the wiper has become necessary to remove sticky pigment ink residue from the wiper. In the early printers using these pigment based inks, this secondary wiper cleaning operation was accomplished using a rigid plastic scraper. Through relative motion of either the scraper, the wiper blade, or both, the wiper was scraped across the plastic cleaner to remove ink from the surfaces of the wiper blade. Unfortunately, the pigment-based ink residue would accumulate on the wiper surface in the form of a paste, which the earlier plastic scraper was not totally effective in removing. Instead, when encountering this paste-like consistency of ink residue, the plastic scraper tended to smear the ink on the surface of the wiper, rather than removing it. Another drawback of the plastic scraper is the tendency of the wiper blade when moving past the scraper to flick ink off of the cleaning surface.
As the inkjet industry investigates new printhead designs, the tendency is toward using permanent or semi-permanent printheads in what is known in the industry as an "off-axis" printer. In an off-axis system, the printheads carry only a small ink supply across the printzone, with this supply being replenished through tubing that delivers ink from an "off-axis" stationary reservoir placed at a remote stationary location within the printer. There are a variety of advantages associated with these off-axis printing systems, but the permanent or semi-permanent nature of the printheads requires special considerations for servicing, particularly when wiping ink residue from the printheads, which must be done without any appreciable wear that could decrease printhead life. To accomplish this objective, an ink solvent has been used in an off-axis printer, specifically the DeskJet 2000C color inkjet printer, sold by the present assignee Hewlett-Packard Company. In this ink solvent system, a polyethylene glycol ("PEG") compound is stored in a porous medium such as a plastic or foam block that is in intimate contact with a reservoir, with this porous block having an applicator portion exposed so the elastomeric wiper can contact the applicator. This elastomeric wiper moves across the applicator to collect PEG, which is then wiped across the printhead to dissolve accumulated ink residue and to deposit a non-stick coating of PEG on the printhead face to retard further collection of ink residue. The wiper then moves across a rigid plastic scraper to remove dissolved ink residue and dirtied PEG from the wiper before beginning the next wiping stroke. The PEG fluid also acts as a lubricant, so the rubbing action of the wiper does not unnecessarily wear the printhead.
Other wiper scraper systems without a solvent have also been sold by the Hewlett-Packard Company in the DeskJet 850C, 855C, 870C 890C and 895C models of color inkjet printers. These scraper systems used a rotary tumbler to scrape the each wiper across a single, associated, cammed scraper. Another wiper system is shown in U.S. Pat. No. 5,815,176. An additional solventless wiper scraper system has been sold by the present assignee, the Hewlett-Packard Company, in the DeskJet 720C and 722C models of inkjet printers, which used a translating pallet to move the wipers into contact with a single stationary scraper bar. Another system having fabric-lined or bristle-lined wiper scrapers has also been proposed. Unfortunately, both the scraper systems that use an ink solvent, and those that do not, tended to flick ink residue into undesirable locations, such as along the side of the printhead and along the interior walls of the service station. In some cases, the ink residue landed in the printhead caps for other colors, leading to cross contamination and mixed colors when printing, which is then manifested as poor print quality. In other instances, the residue was flicked onto the service station gear mechanism, where it fouled the gear operation, or onto a cartridge's electrical interconnect with the carriage where it often promoted shorts. Moreover, this flicking action, which occurs after scraping when the wiper snaps back to an upright position, also generates undesirable noise as the wipers snap off the scraper at high speeds and then vibrate to an eventual stop.
Thus, a need exists for an inkjet printhead cleaning system which scrapes ink residue and ink solvent from the wiper while minimizing ink flicking from the wiper blade, and which is quieter than the earlier wiper scraper designs.